MAP kinase phosphatase-1 gene transcription in rat neuroendocrine cells is modulated by a calcium-sensitive block to elongation in the first exon.

Transcriptional elongation of many eukaryotic, prokaryotic, and viral genes is tightly controlled, which contributes to gene regulation. Here we describe this phenomenon for the MAP kinase phosphatase 1 (MKP-1) immediate early gene. In rat GH4C1 pituitary cells, MKP-1 mRNA is rapidly and transiently induced by the thyrotropin-releasing hormone (TRH) and the epidermal growth factor EGF via transcriptional activation of the gene. Ca(2+) signals are necessary for the induction of MKP-1 in response to TRH but not to EGF. Reporter gene analysis with the newly cloned rat promoter sequence shows only limited induction in response to various stimuli, including TRH or EGF. By nuclear run-on assays we demonstrate that in basal conditions, a strong block to elongation in the first exon regulates the MKP-1 gene and that stimulation with either TRH or EGF overcomes the block. Ca(2+) signals are important to release the MKP-1 elongation block in a manner similar to the c-fos oncogene. These results suggest that a common mechanism of intragenic regulation may be conserved between MKP-1 and c-fos in mammalian cells.

Essentiality of intron control in the induction of c-fos by glucose and glucoincretin peptides in INS-1 beta-cells.

Glucose controls long-term processes in the pancreatic beta-cell such as metabolic enzymes gene expression, cell growth, and apoptosis. Such control is likely mediated via the expression of immediate-early response genes since several of these genes including c-fos are strongly induced by glucose in the beta-cell line INS-1, provided costimulation with cAMP-raising glucoincretin hormones. This study addresses the mechanism of c-fos gene activation by glucose. Glucose in the presence of chlorophenylthio-cAMP generated a low threefold induction of the c-fos/basic luciferase reporter gene, which includes only the c-fos promoter. In contrast, the c-fos/intron construct containing the first intron in addition to promoter elements showed a pronounced 16-fold induction, comparable to the increased c-fos mRNA accumulation. Similar observations were made with glucose in combination with the glucoincretins glucagon-like peptide 1, glucose-dependent insulinotropic polypeptide, and pituitary adenylyl cyclase-activating peptide 38. Deletion of a 119 bp region in intron 1 that includes a transcriptional arrest site did not affect the inductive process. In contrast, a 534 bp deletion comprising a major part of the intron reduced the induction by 75%. At the promoter level, mutating the cAMP response element reduced by more than 60% the transcriptional activation whereas mutating the serum response element had no effect. Inhibitors of protein kinase A and Ca(2+)/calmodulin-dependent protein kinases each reduced by 50% the reporter gene activation and together fully prevented the glucose-glucoincretin effect. In conclusion, the strong induction of c-fos by glucose and glucoincretins results from Ca(2+) and cAMP signaling pathways addressing both the CRE in the promoter and essential response element(s) in the first intron that are unrelated to the transcription arrest site.

Calcium signalling and gene expression.

A wide variety of compounds acting as extracellular signals cause changes in the free cytosolic Ca2+ concentration. These factors include hormones, growth factors, neurotransmitters, but also nutrient and metabolic activators. Ca2+ signalling is caused by mobilization of Ca2+ from internal stores and by well controlled and timed Ca2+ influx from the extracellular space. Ca2+ signals address Ca2+ dependent enzymes, most importantly Ca2+ sensitive protein kinases and phosphatases. The profound influence of Ca2+ signalling on gene expression has been recognized a long time ago. As Ca2+ signals are short-lived when compared to alterations in differentiated gene expression, it is generally considered that genes coding for short-lived transcription factors (i.e. fos, jun) are the immediate target of Ca2+ signalling. Transcription of these immediate early genes (IEG) can be activated without the need for protein synthesis. Ca2+ signalling affects differentiated gene expression via changes in the absolute and relative abundance of IEG products, which in turn control the expression of differentiated genes. Ca2+ signals can stimulate both transcriptional initiation as well as transcriptional elongation. Initiation of transcription is stimulated by the Ca2+ dependent phosphorylation of binding proteins addressing two response elements in the promoter of IEGs: the cAMP response element, CRE, and the serum response element, SRE. Distinct protein kinases are involved in either case. We study the elongation of transcripts of the IEG c-fos beyond the first intron which is favoured by Ca2+ signals, involving mechanisms which still are poorly understood. We can show that intron sequences contribute to the control of elongation by Ca2+, and that there is a strong interrelation between the transcription control by the promoter and by the intron.

Characterization of iodothyronine sulfotransferase activity in rat liver.

Sulfation is an important pathway in the metabolism of thyroid hormone because it strongly facilitates the degradation of the hormone by the type I iodothyronine deiodinase. However, little is known about the properties and possible regulation of the sulfotransferase(s) involved in the sulfation of thyroid hormone. We have developed a convenient method for the analysis of iodothyronine sulfotransferase activity in tissue cytosolic fractions, using radioiodinated 3,3'-diiodothyronine (3,3'-T2) as the preferred substrate, unlabeled 3'-phosphoadenosine-5'-phosphosulfate (PAPS) as the sulfate donor, and Sephadex LH-20 minicolomns for separation of the products. We found that iodothyronine sulfotransferase activity in rat liver cytosol is 1) higher in male than in female rats; 2) optimal at pH 8.0; 3) characterized (at 50 microM PAPS and pH 7.2) by apparent Michaelis-Menton (Km) values for 3,3'-T2 of 1.77 and 4.19 microM, and Vmax values of 1.94 and 1.45 nmol/min per mg protein in male and female rats, respectively; 4) characterized (at 1 microM 3,3'-T2 and pH 7.2) by apparent Km values for PAPS of 4.92 and 3.80 microM and Vmax values of 0.72 and 0.31 nmol/min per mg protein, in males and females, respectively; 5) little affected by hyperthyroidism in both male and female rats, but significantly decreased by hypothyroidism in males but not in females; and 6) not affected by short-term (3 days) fasting in both male and female rats, but significantly decreased by long-term (3 weeks) food restriction to one-third of normal intake in males but not in females. It is suggested that the higher hepatic iodothyronine sulfotransferase activity in male vs. female rats, as well as the decreases induced in males by hypothyroidism and long-term food restriction, represents differences in the expression of the male-dominant isoenzyme rSULT1C1.

Possible role of corticosterone in the down-regulation of the hypothalamo-hypophysial-thyroid axis in streptozotocin-induced diabetes mellitus in rats.

We investigated the effects of diabetes mellitus on the hypothalamo-hypophysial-thyroid axis in male (R x U) F1 and R-Amsterdam rats, which were found to respond to streptozotocin (STZ)-induced diabetes mellitus with no or marked increases, respectively, in plasma corticosterone. Males received STZ (65 mg/kg i.v.) or vehicle, and were killed 1, 2 or 3 weeks later. At all times studied, STZ-induced diabetes mellitus resulted in reduced plasma TSH, thyroxine (T4) and 3,5,3'-tri-iodothyronine (T3). Since the dialyzable T4 fraction increased after STZ, probably as a result of decreased T4-binding prealbumin, plasma free T4 was not altered during diabetes. In contrast, both free T3 and its dialyzable fraction decreased during diabetes, which was associated with an increase in T4-binding globulin. Hepatic activity of type I deiodinase decreased and T4 UDP-glucuronyltransferase increased after STZ treatment. Thus, the lowered plasma T3 during diabetes may be due to decreased hepatic T4 to T3 conversion. Median eminence content of TRH increased after STZ, suggesting that hypothalamic TRH release is reduced during diabetes and that this is not caused by impaired synthesis or axonal transport of TRH to the median eminence. Hypothalamic proTRH mRNA did not change in diabetic (R x U) F1 rats during the period of observation, but was lower in R-Amsterdam rats 3 weeks after STZ. Similarly, pituitary TSH and TSH beta mRNA had decreased in R-Amsterdam rats by 1 week after STZ treatment, but did not change in (R x U) F1 rats. The difference between the responses in diabetic R-Amsterdam and (R x U) F1 rats may be explained on the basis of plasma corticosterone levels which increased in R-Amsterdam rats only. Hypothalamic TRH content was not affected by diabetes mellitus, but the hypothalami of diabetic rats released less TRH in vitro than those of control rats. Moreover, insulin had a positive effect on TRH release in vitro. In conclusion, the reduced hypothalamic TRH release during diabetes is probably not caused by decreases in TRH synthesis or transport to the median eminence, but seems to be due to impaired TRH release from the median eminence which may be related to the lack of insulin. Inhibition of proTRH and TSH beta gene expression in diabetic R-Amsterdam rats is not a primary event but appears to be secondary to enhanced adrenal activity in these animals during diabetes.

Gender-specific changes in thyroid hormone-glucuronidating enzymes in rat liver during short-term fasting and long-term food restriction.

Glucuronidation is a major pathway of thyroid hormone metabolism in rats, involving at least three different hepatic UDP-glucuronyltransferases (UGTs): bilirubin UGT, phenol UGT and androsterone UGT. We have studied the effects of short-term (3 days) fasting and long-term (3 weeks) food restriction to one-third of normal intake (FR33) on hepatic UGT activities for thyroxine (T4), triiodothyronine (T3), bilirubin and androsterone in male and female Wistar rats with either a functional (high activity, HA) or a defective (low activity, LA) androsterone UGT gene. Because food deprivation is known to induce centrally mediated hypothyroidism in rats, results were compared with those obtained in methimazole (MMI)-induced hypothyroid rats. Both fasting and FR33 produced largely parallel increases in T4 and bilirubin UGT activities. These effects were greater in males than in females, and were reproduced in MMI-treated rats. In male and female HA rats, fasting induced insignificant increases in T3 UGT activity and had no effect on androsterone UGT activity. In male HA rats, FR33 was associated with an increase in T3 UGT activity, while androsterone UGT activity showed little change. However, in female HA rats both T3 and androsterone UGT activities were markedly decreased by FR33. Triiodothyronine UGT activity in LA rats was strongly decreased compared with HA rats, but was not further decreased by FR33 in female LA rats, supporting the importance of androsterone UGT for T3 glucuronidation. These results demonstrate different sex-dependent effects of food deprivation on hepatic T4 and T3 glucuronidation that are associated with changes in the expression of bilirubin UGT and androsterone UGT, respectively. For the increased T4 and bilirubin UGT activities at least, these effects appear to be mediated by the hypothyroid state of the (semi)starved animals.

Effects of long-term food reduction on the hypothalamus-pituitary-thyroid axis in male and female rats.

The reduced thyroid activity during short-term starvation is associated with a lowered hypothalamic synthesis and secretion of TRH. However, little is known about the cause of the reduced thyroid function during prolonged malnutrition. We have therefore studied the effects of food reduction to one-third of normal (FR33) on the hypothalamus-pituitary-thyroid axis of male and female Wistar rats. After 3 weeks body weights of FR33 rats were almost 50% lower than those of controls. In both sexes, FR33 caused marked increases in serum corticosterone, and decreases in serum TSH, thyroxine (T4), free T4, tri-iodothyronine (T3) and free T3. While the free T3 fraction (FFT3) in serum decreased, the free T4 fraction (FFT4) tended to increase. Electrophoretic analysis indicated that decreased FFT3 was correlated with an increased thyroxine-binding globulin, while the increase in FFT4 seemed due to a decreased thyroxine-binding prealbumin binding capacity. Total RNA and proTRH mRNA in the hypothalamus were not affected by FR33. Median eminence and posterior pituitary TRH content tended to increase in FR33 rats, suggesting that hypothalamic TRH release is reduced in FR33 rats. Anterior pituitary TSH content was decreased by FR33 in both sexes, but pituitary TSH beta mRNA and TRH receptor status were not affected except for increased pituitary TSH beta mRNA in female FR33 rats. Although FR33 had no effect on pituitary weight, pituitary RNA and membrane protein content in FR33 rats were 50-70% lower than values in controls. In conclusion, prolonged food reduction suppresses the pituitary-thyroid axis in rats. In contrast to short-term food deprivation, the mechanism whereby serum TSH is suppressed does not appear to involve decreases in proTRH gene expression, but may include effects on pituitary mRNA translation. Our results further support the hypothesis that TSH release may be lowered by increased corticosterone secretion, although the mechanism of this effect may differ between acute starvation and prolonged food reduction.

Studies on the role of TRH and corticosterone in the regulation of prolactin and thyrotrophin secretion during lactation.

This study describes the effects of litter size and acute suckling on the synthesis and release of hypothalamic TRH, as indirectly estimated by determination of hypothalamic prothyrotrophin-releasing hormone (proTRH) mRNA and median eminence TRH content. The effects of litter size (five or ten pups) were studied throughout lactation, while suckling-induced acute changes were analyzed on day 13 of lactation in dams with ten pups. In view of the enhanced adrenal activity during lactation and recent evidence that corticosteroids have negative effects on hypothalamic TRH, we also studied adrenalectomized (ADX) dams treated with corticosterone to maintain basal plasma corticosterone levels. In addition to an increased plasma level of prolactin (PRL), adrenal weight and plasma corticosterone increased, while plasma TSH, tri-iodothyronine (T3), thyroxine (T4) and free T4 (FT4) levels decreased during lactation. Litter size correlated positively with plasma PRL, adrenal weight and plasma corticosterone. No effect of litter size was observed on plasma T3, but rats with ten pups had lower plasma TSH, T4 and FT4 than rats with a five-pup litter. Compared with dioestrous rats, lactating rats showed an increased hypothalamic proTRH mRNA content on day 2, but not on days 8 and 15 of lactation. Median eminence TRH in lactating rats gradually increased until day 15 and decreased thereafter. Acute suckling, after a 6-h separation of mother and pups, rapidly increased plasma PRL and corticosterone in the mothers, but had no effects on plasma TSH and thyroid hormone levels. Hypothalamic proTRH mRNA increased twofold after 0.5 h of suckling, and then gradually returned to presuckling values after 6 h. Compared with sham-operated rats, corticosterone-substituted ADX rats with ten pups had increased plasma PRL and TSH, hypothalamic proTRH mRNA and pituitary TSH beta mRNA on day 15 of lactation. Moreover, while acute suckling did not enhance TSH release in sham-operated rats, it provoked not only PRL but also TSH release in corticosterone-substituted ADX dams. It is concluded that suckling exerts a rapid, positive effect on hypothalamic proTRH mRNA content. However, the concurrent enhanced adrenal activity has negative effects on hypothalamic proTRH gene expression resulting in a suppressed hypophysial-thyroid axis during lactation. While TRH appears to play a role in PRL release during the first days of lactation and during acute suckling, TRH seems not important in maintaining PRL secretion during continued suckling.

Further studies on the regulation, localization and function of the TRH-like peptide pyroglutamyl-glutamyl-prolineamide in the rat anterior pituitary gland.

Recent evidence shows that thyrotrophin-releasing hormone (TRH) immunoreactivity in the rat anterior pituitary gland is accounted for by the TRH-like tripeptide pyroglutamyl-glutamyl-prolineamide (pGlu-Glu-ProNH2, < EEP-NH2). The present study was undertaken to investigate further the regulation, localization and possible intrapituitary function of < EEP-NH2. Anterior pituitary levels of < EEP-NH2 were determined between days 5 and 35 of life, during the oestrous cycle and after treatment with the luteinizing hormone-releasing hormone (LHRH) antagonist Org 30276. Treatment of adult males with the LHRH antagonist either for 1 day (500 micrograms/100 g body weight) or for 5 days (50 micrograms/100 g body weight) reduced anterior pituitary < EEP-NH2 levels by 25-30% (P < 0.05 versus saline-treated controls). Anterior pituitary < EEP-NH2 increased between days 5 and 35 of life. In females, these levels were 2- to 3-fold higher (P < 0.05) than in males between days 15 and 25 after birth; these changes corresponded with the higher plasma follicle-stimulating hormone (FSH) levels in the female rats. After day 25, < EEP-NH2 levels in female rats decreased in parallel with a decrease in plasma FSH. Injections with the LHRH antagonist (500 micrograms/100 g body weight), starting on day 22 of life, led to reduced contents of < EEP-NH2 in the anterior pituitary gland of female rats on days 26 and 30 (55 and 35% decrease respectively). Levels of < EEP-NH2 in the anterior pituitary gland did not change significantly during the oestrous cycle.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation of thyrotropin-releasing hormone in the posterior pituitary.

Although the presence of thyrotropin-releasing hormone (TRH) in the posterior pituitary (PP) was reported more than one decade ago, knowledge on its origin, regulation and functional significance is lacking. In the present study we investigated the regulation of TRH in the rat PP. Analysis by specific RIA, anion and cation exchange chromatography and reverse-phase HPLC showed that all TRH immunoreactivity in the PP is accounted for by authentic TRH. Induction of hyperthyroidism with thyroxine increased levels of TRH in the PP by 20%, whereas in methimazole-treated, hypothyroid rats the content decreased by 25% versus untreated, euthyroid controls. Food deprivation for 3 days increased levels by 35% and refeeding completely normalized TRH content again. Also 14-17 days after castration, TRH in the PP was increased by 25% while testosterone substitution prevented this increase. Castration did not affect proTRH mRNA levels in the hypothalamus. One week after adrenalectomy or daily subcutaneous dexamethasone injections, TRH content in the PP was not affected. Treatment with disulfiram, an inhibitor of the peptidylglycine alpha-amidating monooxygenase (PAM), reduced levels of TRH in the PP by 20%. ProTRH and PAM mRNA levels were not affected in the hypothalamus by this treatment. Since TRH in the PP has been suggested to play a role in prolactin (PRL) release, we determined the content of TRH in the PP during a 6-hour suckling stimulus that increased PRL levels in peripheral blood 30-fold. Whereas TRH in the median eminence increased by 35%, 6 h after the initiation of suckling, TRH levels in the PP remained constant.(ABSTRACT TRUNCATED AT 250 WORDS)

Starvation-induced changes in the hypothalamic content of prothyrotrophin-releasing hormone (proTRH) mRNA and the hypothalamic release of proTRH-derived peptides: role of the adrenal gland.

The purpose of this study was to investigate the mechanisms involved in the reduced thyroid function in starved, young female rats. Food deprivation for 3 days reduced the hypothalamic content of prothyrotrophin-releasing hormone (proTRH) mRNA, the amount of proTRH-derived peptides (TRH and proTRH160-169) in the paraventricular nucleus, the release of proTRH-derived peptides into hypophysial portal blood and the pituitary levels of TSH beta mRNA. Plasma TSH was either not affected or slightly reduced by starvation, but food deprivation induced marked increases in plasma corticosterone and decreases in plasma thyroid hormones. Refeeding after starvation normalized these parameters. Since the molar ratio of TRH and proTRH160-169 in hypophysial portal blood was not affected by food deprivation, it seems unlikely that proTRH processing is altered by starvation. The median eminence content of pGlu-His-Pro-Gly (TRH-Gly, a presumed immediate precursor of TRH), proTRH160-169 or TRH were not affected by food deprivation. Since median eminence TRH-Gly levels were very low compared with other proTRH-derived peptides it is unlikely that alpha-amidation is a rate-limiting step in hypothalamic TRH synthesis. Possible negative effects of the increased corticosterone levels during starvation on proTRH and TSH synthesis were studied in adrenalectomized rats which were treated with corticosterone in their drinking water (0.2 mg/ml). In this way, the starvation-induced increase in plasma corticosterone could be prevented. Although plasma levels of thyroid hormones remained reduced, food deprivation no longer had negative effects on hypothalamic proTRH mRNA, pituitary TSH beta mRNA and plasma TSH in starved adrenalectomized rats. Thus, high levels of corticosteroids seem to exert negative effects on the synthesis and release of proTRH and TSH. This conclusion is corroborated by the observation that TRH release into hypophysial portal blood became reduced after administration of the synthetic glucocorticosteroid dexamethasone. On the basis of these results, it is suggested that the reduced thyroid function during starvation is due to a reduced synthesis and release of TRH and TSH. Furthermore, the reduced TRH and TSH synthesis during food deprivation are probably caused by the starvation-induced enhanced adrenal secretion of corticosterone.

Regulation of the TRH-like peptide pyroglutamyl-glutamyl-prolineamide in the rat anterior pituitary gland.

TRH-like peptides share the N- and C-terminal amino acids with TRH (pGlu-His-Pro-NH2) but differ in the middle amino acid residue. One of them, pGlu-Glu-Pro-NH2 (< EEP-NH2; EEP) is present in the rat pituitary gland, but its biological significance is unknown. We investigated the localization and regulation of this tripeptide in the rat pituitary gland. To distinguish between TRH and EEP two antisera were used for RIA: specificity of antiserum 4319 for the TRH-like peptides pGlu-Phe-Pro-NH2 and EEP was equal to or greater than that for TRH, whereas antiserum 8880 is TRH-specific. Our RIA data showed the presence of a TRH-like peptide in the anterior pituitary gland (AP) and of TRH in the posterior pituitary gland (PP). The TRH-like peptide in the AP was identified on anion-exchange chromatography and subsequent HPLC as EEP. Pathophysiological conditions such as altered thyroid and adrenal status and suckling did not affect pituitary gland levels of EEP. In general, however, there is a clear sex difference: levels of EEP are higher in male than in female rats. In both sexes gonadectomy leads to a substantial two- to threefold rise in EEP levels, abolishing the sex difference. Testosterone administration to gonadectomized male rats normalizes levels of EEP again. Disulfiram, an inhibitor of the enzyme peptidylglycine alpha-amidating monooxygenase, reduced levels of EEP in the AP by approximately 50%.(ABSTRACT TRUNCATED AT 250 WORDS)

Different effects of continuous infusion of interleukin-1 and interleukin-6 on the hypothalamic-hypophysial-thyroid axis.

The cytokines interleukin-1 (IL-1) and IL-6 are thought to be important mediators in the suppression of thyroid function during nonthyroidal illness. In this study we compared the effects of IL-1 and IL-6 infusion on the hypothalamus-pituitary-thyroid axis in rats. Cytokines were administered by continuous ip infusion of 4 micrograms IL-1 alpha/day for 1, 2, or 7 days or of 15 micrograms IL-6/day for 7 days. Body weight and temperature, food and water intake, and plasma TSH, T4, free T4 (FT4), T3, and corticosterone levels were measured daily, and hypothalamic pro-TRH messenger RNA (mRNA) and hypophysial TSH beta mRNA were determined after termination of the experiments. Compared with saline-treated controls, infusion of IL-1, but not of IL-6, produced a transient decrease in food and water intake, a transient increase in body temperature, and a prolonged decrease in body weight. Both cytokines caused transient decreases in plasma TSH and T4, which were greater and more prolonged with IL-1 than with IL-6, whereas they effected similar transient increases in the plasma FT4 fraction. Infusion with IL-1, but not IL-6, also induced transient decreases in plasma FT4 and T3 and a transient increase in plasma corticosterone. Hypothalamic pro-TRH mRNA was significantly decreased (-73%) after 7 days, but not after 1 or 2 days, of IL-1 infusion and was unaffected by IL-6 infusion. Hypophysial TSH beta mRNA was significantly decreased after 2 (-62%) and 7 (-62%) days, but not after 1 day, of IL-1 infusion and was unaffected by IL-6 infusion. These results are in agreement with previous findings that IL-1, more so than IL-6, directly inhibits thyroid hormone production. They also indicate that IL-1 and IL-6 both decrease plasma T4 binding. Furthermore, both cytokines induce an acute and dramatic decrease in plasma TSH before (IL-1) or even without (IL-6) a decrease in hypothalamic pro-TRH mRNA or hypophysial TSH beta mRNA, suggesting that the acute decrease in TSH secretion is not caused by decreased pro-TRH and TSH beta gene expression. The TSH-suppressive effect of IL-6, either administered as such or induced by IL-1 infusion, may be due to a direct effect on the thyrotroph, whereas additional effects of IL-1 may involve changes in the hypothalamic release of somatostatin or TRH.(ABSTRACT TRUNCATED AT 400 WORDS)

Effect of starvation and subsequent refeeding on thyroid function and release of hypothalamic thyrotropin-releasing hormone.

Effects of starvation on thyroid function were studied in 5- to 6-week-old (R x U) F1 rats. Starvation lowered plasma TSH in female, but not in male rats. Plasma T4 and T3 levels decreased, whereas the dialysable T4 fraction increased during starvation. Free T4 (FT4) levels decreased rapidly in females, but only after prolonged fasting in male rats. Glucose decreased, and free fatty acid levels increased during starvation. Peripheral TRH levels did not change during food deprivation. Since effects of starvation were most apparent in young female rats, such rats were used to study hypothalamic TRH release during starvation and subsequent refeeding. Basal in vitro hypothalamic TRH secretion was less in starved rats than in control or refed animals. In vitro hypothalamic TRH release in medium with 56 mM KCl increased 3-fold compared to basal release, and in these depolarization conditions TRH release was similar between hypothalami from control, starved and refed rats. In rats starved for 2 days, TRH level in hypophysial portal blood was lower than that of controls. Thus, diminished thyroid function during starvation may at least in part be caused by a reduced hypothalamic TRH release.

Regulation of hypothalamic TRH production and release in the rat.

Thyrotropin-releasing hormone (TRH) is produced in the hypothalamic paraventricular nucleus (PVN) as a 255-amino acid precursor (pro-TRH) with 5 TRH progenitor sequences. Pro-TRH is enzymatically processed to yield TRH and other peptides, which are transported to the median eminence and released into hypophysial portal blood. To elucidate the role of TRH in the control of thyroid function, we studied hypothalamic TRH synthesis and release in many conditions. TRH synthesis and release were assessed by pro-TRH mRNA measurement, and by sampling portal blood or push-pull perfusate, respectively. Destruction of the PVN reduced TRH and TSH secretion dramatically, while electrical stimulation of this nucleus enhanced their release. Hence, the PVN is important for normal TSH secretion. TRH synthesis and release decreased in hyperthyroid rats, but increased in hypothyroid rats. The magnitude of these changes, however, was small compared with alterations in TSH, suggesting that the feedback of thyroid hormones on TSH release is mainly exerted at the pituitary level. TRH synthesis and release increased during cold exposure, and decreased during starvation and diabetes. Thus, altered thyroid function during cold exposure, diabetes and starvation seems due to modified hypothalamic TRH synthesis and release.